A printer incorporating a conventional wire dot print head has been used widely due to its advantages, which include a high option among various recording media and the capability of using it with copying paper as its recording medium. The wire dot print head drives the wires by the magnetic attraction of permanent magnets or electromagnets.
Recently, the so-called spring-charged wire dot print head has been employed in most printers due to its high response speed.
The spring-charged wire dot print head is provided with armatures, each fixedly holding a printing wire and being supported by a biasing flat spring adapted for swinging motion. The armature is attracted against the resilience of the biasing flat spring to a core by the magnetic attraction of a permanent magnet. In printing, a coil wound around the core is energized to release the armature from the permanent magnet by establishing in the coil a magnetic flux of a polarity reverse to that of the permanent magnet.
In the spring-charged wire dot print head, it is possible that leakage flux among a magnetic flux produced by the electromagnet for cancelling the magnetic flux produced by the permanent magnet causes magnetic interference with, the magnetic flux in the adjacent armature and core, thereby causing change in the magnetic flux in the adjacent armature and core. The effect of magnetic interference on the change of magnetic flux increases with the number of printing wires simultaneously driven for printing, and each coil requires an excitation current greater than that necessary for releasing the corresponding armature from the core when the printing wire is driven individually, thereby increasing the power consumption and rate of heat generation for the printing head.
Since variation in the exciting current affects the action of the released armature, the duration of the supply of current to the coil must be controlled according to the number of printing wires to be driven simultaneously for printing.
The power consumption and heat generation of the spring-charged wire dot print head are further increased by magnetic interference particularly when the spring-charged wire dot print head is miniaturized, formed in a compact construction and operated at a high printing speed.
Many, improvements have been developed to solve such problems. Japanese Patent Laid-open Publication No. 58-96568 discloses a wire dot print head which attempt to account for the magnetic interference by magnetizing the adjacent cores respectively in opposite polarities. This known wire dot print head is shown in FIGS. 1 to 3. FIG. 1 is a sectional view of this known wire dot print head, FIG. 2 is a sectional view taken along the line A--A in FIG. 1, and FIG. 3 is a perspective view of an essential portion of the wire dot print head of FIG. 1.
Referring to FIGS. 1 to 3, a circular bottom frame 11 is formed of a nonmagnetic material, such as aluminum. A plurality of cores 12 having a shape substantially resembling the letter L are placed on the bottom frame 11 in a radial arrangement with their upright portions adjacent the center of the print head. Coils 13 are wound around the upright portions of the cores 12 to form electromagnets 14. Permanent magnets 15 are placed respectively on the rear ends of the cores 12, namely, portions of the cores 12 near the circumference of the print head. The respective polarities of the permanent magnets 15 on the adjacent cores 12 are opposite to each other.
Side yokes 16 are placed respectively on the permanent magnets 15. Flat springs 17 are disposed with their free ends positioned opposite to the corresponding electromagnets 14. Armatures 18 are fastened respectively to the free ends of the flat springs 17. Upper yokes 19 are placed on the flat springs 17. A top frame 20 formed of a nonmagnetic material, such as aluminum, is placed on the upper yokes 19. The top frame 20 is provided integrally with a wire guide 21 in its central portion to hold the tips of printing wires 22 in a predetermined arrangement and to guide the same. The side yokes 16 placed on the permanent magnets 15, the flat springs 17, the upper yokes 19 and the top frame 20 are fastened together with screws 23.
The actions of the dot print head thus constructed will be described hereinafter.
When inoperative, the permanent magnet 14 is not excited and the magnetic flux produced by the permanent magnet 15 passes through the side yoke 16, the upper yoke 19, the armature 18 and the core 12 in that order as indicated by an arrow e. Therefore, the armature 18 is attracted to the core 12 against the resilience of the flat spring 17, so that the flat spring 17 is biased so as to retract the printing wire 22.
In performing a printing operation by selectively driving the printing wires 22, the coil 13 corresponding to the printing wire 22 to be driven for printing is energized. Then, a magnetic flux of a polarity opposite to that of the permanent magnet 15 passed through the armature 17, the upper yoke 19 and the side yoke 16 in that order as indicated by arrows f and g to cancel the magnetic flux indicated by the arrow e, whereby the armature 18 is released from the core 12. Consequently, the printing wire 22 is advanced by the stored energy of the flat spring 17 to print a dot on the recording medium. The printing wires 22 are thus driven selectively to print characters with dot matrices.
The polarity of the magnetic flux indicated by the arrow g is opposite to that of the magnetic flux in the adjacent permanent magnet 15 indicated by an arrow h, and the magnetic flux produced by the electromagnet 14 cancels the magnetic flux produced by the adjacent permanent magnet 15. Therefore, when the adjacent coils 13 are energized simultaneously, the magnetic flux produced by one of the adjacent coils 13 cancels the magnetic flux produced by the permanent magnet 15 corresponding to the other coil 13 and vice versa, and hence the electromagnets 14 can be magnetized satisfactorily by supplying a comparatively small exciting current to the coils 13. Thus, the wire dot print head operates at a comparatively low power consumption rate.
This known wire dot print head, however, places a restriction on the manufacturing process. Since the respective polarities of the adjacent, individual permanent magnets 15 corresponding to the printing wires 22 are opposite to each other, it is impossible to magnetize the permanent magnets 15 simultaneously in a magnetic field of an optional intensity after assembling the wire dot print head. Therefore, the permanent magnets 15, magnetized beforehand in opposite polarities in a desired magnetization intensity, must be arranged individually in assembling the wire dot.,print head through a complicated manufacturing process which is difficult to control. Furthermore, the flat springs 17, the side yokes 16 and the upper yokes 19, in addition to the permanent magnets 15, must be manufactured individually, which increases the cost of the wire dot print head.
Accordingly, it is an object of the present invention to solve the problem inherent in the conventional wire dot print head and to provide a wire dot print head capable of being manufactured by a simple manufacturing process and of operating at a comparatively low power consumption rate.
It is another object of the present invention to provide a wire dot print head capable of stable performance without being affected by different magnetic path configurations.